The high speed of bipolar integrated circuits has been a major attraction to circuit designers in both logic and memory applications. Performance of bipolar circuits has been enhanced by a combination of down-scaling and the use of advanced processing methods. This advanced performance has, however, been accompanied by a greater susceptibility to soft-error caused by .alpha.-particle radiation.
A major candidate for high speed static bipolar memory cells has been the Emitter Coupled Logic (ECL) cell. U.S. Pat. No. 4,255,674, filed Jan. 2, 1979, shows a cell of this type wherein one of the emitter regions of each transistor of the flip-flop of the cell is connected to a supply line and the other emitter regions are connected to read/write lines. The discussion in the specification of this reference characterizes the supply as a current source G. The latter is the only current source disclosed and appears to be that current source which is connected to and shared by all cells connected to a common word line.
A later reference, U.S. Pat. No. 4,314,359, directly addresses the problem of erroneous operation of ECL-type memory cells due to .alpha.-particles. In addition to describing ECL bipolar memory cells of the type shown in the above, first mentioned reference, it discusses in detail the emitters of the cell which are commonly connected to a data-holding constant current source I.sub.ST. The latter is the same as current source IR.sub.1 shown in FIG. 3. IR.sub.1 is shown connected to word line WL1 which serves a plurality of ECL memory cells MS.sub.11 -MS.sub.12. There is then no separate constant current source in each individual cell. As a result, the capacitance associated with word line W.sub.12 which is quite large, ties down the potential of emitters C.sub.2 and C.sub.4. The approach of the reference to overcoming soft error due to .alpha.-particle impingement is to include additional capacitance between the collector and base regions of each transistor pair of each memory cell.
In a more recent reference, U.S. Pat. 4,541,003, filed June 14, 1982, an ECL cell is shown in FIG. 23 and the comment is made that electron-hole pairs formed by irradiation of .alpha.-particles easily causes the state of the flip-flop to be reversed, resulting in the occurrence of soft error. Such errors are eliminated in the reference by means of a semiconductor shielding element. A still more recent reference entitled "A 3.5NS 2W 20 mm.sup.2 16Kb bipolar RAM" by K. Yamaguchi et al, IEEE Dig. of ISSCC, 1986, pp. 214-215 shows the incorporation of a high capacitance tantalum pentoxide (Ta.sub.2 O.sub.5) film to increase the storage node capacitance.
In yet another reference entitled "A double wordline structure in ECL RAM" by Y. Nakase et al, Dig. of VLSI Tech. Symp. pp. 75-76, 1986, the splitting of a top-word line is used to increase the potential difference between the collector nodes of the flip-flop transistors.
From all the foregoing, it should be clear that none of the above cited references provide an ECL or bipolar static RAM cell wherein the potential of the common-emitter node of the cross-coupled transistors is allowed to swing freely. In all cases, some expedient is utilized which affects word line or device capacitance and where constant current sources are mentioned, such sources are those normally connected to a word line and commonly connected to a plurality of ECL cells which share the same word line.
It is, therefore, a principal object of the present invention to provide a SRAM cell of the ECL type which has a reduced susceptibility to soft errors due to .alpha.-particle radiation.
Another object is to provide a SRAM cell of the ECL type wherein means for reducing soft error due to .alpha.-particle radiation are connected between commonly connected emitters of a cell and its associated word line.
Another object is to provide a SRAM cell of the ECL type wherein a constant current source or a current mirror is connected between commonly connected cell emitters and a word line.
Still another object is to provide a SRAM cell of the ECL type wherein another constant current source or current mirror is connected between commonly connected emitter and a word line to which the usual constant current source is connected.